For the power supply to high value electronic equipment such as computer or telecommunication installations, often two or more mains are switched in parallel to supply the equipment. If one of the mains fails, power is alternately supplied with the other mains. With this it is assured that the voltage or electric power supply of sensitive equipment goes without interruptions, even in critical working phases. Mains in this case are to be understood as electric power supply as well as voltage supply modules, whose primary energy is taken from an alternating or direct current source.
When power is provided by several parallel wired mains, two types of operation have to be distinguished. The first one is in normal operation to activate one main, which supplies the full power to the load, while the other mains are provided as power reserve and stay passive and are only switched on in case of a failure. In this case the active main is subjected to high demands, which increases the risk of failure. With the second operational possibility the power to the load is at the same time distributed over several mains, where the power distribution is divided up by a predetermined code. In general an even load for the mains is strived for. If by a defect, one of the mains breaks down, the power distribution is changed accordingly. This mode of operation has the advantage that the mains in normal operation are only burdened with a fraction of their nominal rating, by which load depending factors which could influence the functional ability of a mains have a small influence on the life of the mains.
Even though a power supply system for safety reasons is equipped with several mains, a dependability is only assured when all sub-assemblies work without faults. This means that if only one of the mains fails, the functional efficiency of the power supply system is limited and the defective mains have to be repaired or replaced with a new one. The mean time between two failures of a power supply system is, according to statistical analysis, directly dependent on the failure probability of each component, meaning the failure probability of dependability of the mains.
From reliability techniques, it is known that the mean time between two failures of mean utilization time of a mains will be shortened out of proportion by increasing the thermal load. The resulting maximum mean utilization time of an electric power supply system will be reached, due to the dependence of the failure probability of each of the individual mains, when the load on the individual mains in the average is at minimum. For this reason the above described second type of power supply system has a longer mean utilization time than the first mentioned one.
From these considerations, with known power supply systems, the total load is distributed over several mains. For this the total current, which sometimes can fluctuate considerably, will be determined and distributed to the mains by a certain ratio. If the mains only produce an output voltage, by dividing the current, the main power is divided in the same proportion. If, however, the mains produce, respectively, several output voltages, for each voltage a separate power distribution of the mains has to be effected, which makes the expenditure for controls very high.
By division of the total current to the parallel, switched mains, it is not yet assured that the power supply system has a small probability of failure, because for the thermal load important influence factors, such as lost power in the mains, converted to heat, the variation or the primary voltage, as well as the installation conditions of the mains, have not been taken under consideration. With unfavorable working conditions of the mains, for example caused by insufficient cooling, high ambient temperatures of different heat exchange resistance between the heat source and heat sink, it can occur that the mains with even power distribution are being utilized differently and as a consequence have a higher risk of failure. This can lead to a decrease of the average utilization time of the power supply system.
A power supply system with several mains, whose power output is switched parallel and which supplies a common load is known from the magazine Electronic Design, Nov. 14, 1985 pages 125 to 132. The output efficiency of the respective mains depends in this power distribution system on the total to the load giving output, which is provided by a reference voltage and on the other hand by a signal of current sensors with which the needed portion of the mains for the total power is determined. This power distribution system shows the before mentioned disadvantages.
It is also known from the magazine Industrie-elektrikelektronik 1988 No. 3, pages 54 to 55 to limit the output power of several switch regulators with different output voltages, when the appliance temperature exceeds a set limit value. The appliance (device) is being used to its thermal load limit and protected from thermal overload. The average utilization time of the power supply system will not be increased by this.